Solid-state light emitting devices and signage with photoluminescence wavelength conversion

ABSTRACT

A solid-state light emitting device comprises a solid-state light emitter (LED) operable to generate excitation light and a wavelength conversion component including a mixture of particles of a photoluminescence material and particles of a light reflective material. In operation the phosphor absorbs at least a portion of the excitation light and emits light of a different color. The emission product of the device comprises the combined light generated by the LED and the phosphor. The wavelength conversion component can be light transmissive and comprise a light transmissive substrate on which the mixture of phosphor and reflective materials is provided as a layer or homogeneously distributed throughout the volume of the substrate. Alternatively the wavelength conversion component can be light reflective with the mixture of phosphor and light reflective materials being provided as a layer on the light reflective surface. A wavelength conversion component, light emitting sign and light emitting signage surface are also disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/253,031, entitled “Solid-State Light Emitting Devices and Signagewith Photoluminescence Wavelength Conversion,” filed on Oct. 4, 2011,which claims the benefit of priority to U.S. Provisional PatentApplication Ser. No. 61/390,091, entitled “Solid-State Light EmittingDevices and Signage with Photoluminescence Wavelength Conversion,” filedon Oct. 5, 2010 and to U.S. Provisional Patent Application Ser. No.61/427,411, entitled “Solid-State Light Emitting Devices with RemotePhosphor Wavelength Conversion Component”, filed Dec. 27, 2010, whichare hereby incorporated by reference in their entireties.

FIELD

Some embodiments of the invention relate to solid-state light emittingdevices and signage that use photoluminescence wavelength conversion toconvert light generated by a solid-state light emitter to a desiredcolor of light.

BACKGROUND

White light emitting LEDs (“white LEDs”) are known in the art and are arelatively recent innovation. Due to their long operating lifeexpectancy (>50,000 hours) and high luminous efficacy (70 lumens perwatt and higher) high brightness white LEDs are increasingly being usedto replace conventional fluorescent, compact fluorescent andincandescent light sources.

It was not until LEDs emitting in the blue/ultraviolet part of theelectromagnetic spectrum were developed that it became practical todevelop white light sources based on LEDs. As taught, for example inU.S. Pat. No. 5,998,925, white LEDs include one or morephoto-luminescent materials (e.g., phosphor materials), which absorb aportion of the radiation emitted by the LED and re-emit radiation of adifferent color (wavelength). Typically, the LED chip or die generatesblue light and the phosphor(s) absorbs a percentage of the blue lightand re-emits yellow light or a combination of green and red light, greenand yellow light, green and orange or yellow and red light. The portionof the blue light generated by the LED that is not absorbed by thephosphor combined with the light emitted by the phosphor provides lightwhich appears to the human eye as being nearly white in color.

The exact color generated by the LED light is highly dependent upon theamount of light that is emitted by the phosphor material, since it isthe combination of the amount (and wavelength) of the phosphor-emittedlight and the amount (and wavelength) of the residual blue light thatdetermines the color of the resultant light. Therefore, phosphor-basedLED devices that are intended to generate white light will requiresufficient amounts of phosphors to function correctly, since thephosphor-based LED device having insufficient amounts of phosphormaterials will fail to generate white-appearing light.

The problem is that phosphor materials are relatively costly, and hencecorrespond to a significant portion of the costs for producingphosphor-based LED devices. Typically the phosphor material in a LEDlight is mixed with a light transmissive material such as a silicone orepoxy material and the mixture directly applied to the light emittingsurface of the LED die. This results in a small footprint layer ofphosphor materials placed directly on the LED die, that is neverthelessstill costly to produce in part because of the significant costs of thephosphor materials.

As disclosed in United States patent application US 2008/02118992 A1 toLi, it is also known to provide the phosphor material as a layer on, orincorporate the phosphor material within an, optical component that isphysically located remote to the LED die. This typically results in alayer of phosphor materials having a much larger footprint than theapproach described in the preceding paragraph. Because of its largersize, a much greater amount of phosphor is normally required tomanufacture such “remote phosphor” LED devices. As a result, the costsare correspondingly greater as well to provide the increased amount ofphosphor materials needed for such remote phosphor LED devices. Forexample, United States patent application US 2007/0240346 A1, to Li etal., teaches solid-state light emitting signs in which blue light froman LED is used to excite phosphor materials on a light emitting signagesurface to generate a desired color of light. A large quantity of thephosphor materials must normally be present to populate the expanse ofthe light emitting signage surface for the device to produce theappropriate color for its intended light functionality.

Therefore, there is a need for improved approach to implement LEDlighting apparatuses that maintains the desired color properties of thedevices, but without requiring the large quantities of photo-luminescentmaterials (e.g. phosphor materials) that are required in the priorapproaches.

It is an object of some embodiments of the present invention to providea light emitting device, a light emitting sign, a photoluminescencewavelength conversion component and a photoluminescence signage surfacethat in part at least overcomes the limitations of the known devices.

SUMMARY

Embodiments of the invention concern solid-state light emitting devicesand signage comprising one or more solid-state light emitters, typicallyLEDs, that are operable to generate blue light which is used to excite aphotoluminescence wavelength conversion component or a photoluminescencelight emitting signage surface that contain particles of a blue lightexcitable photo-luminescent (e.g. a phosphor material). In accordancewith some embodiments of the invention, and to increasephotoluminescence light generation by the phosphor material, thewavelength conversion component and/or signage surface further comprisesincorporating particles of a light reflective material (also referred toherein as “light scattering material”) with the phosphor material. Theenhanced light generation results from the light reflective materialincreasing the number of collisions of the LED generated light withparticles of the phosphor material, which decreases the amount ofphosphor material usage to generate a selected emission product color.

According to some embodiments of the invention, a light emitting devicecomprises at least one solid-state light emitter operable to generateblue light and a photoluminescence wavelength conversion componentcomprising a mixture of particles of at least one phosphor material andparticles of a light reflective material, wherein the mixture of atleast one phosphor material and light reflective material aredistributed over a large footprint area, e.g. an area of at least 0.8cm². In particular, including particles of a light reflective materialwith the phosphor material can increase photoluminescence lightgeneration by the phosphor material. The increase in photoluminescencelight generation results from the light reflective material increasingthe probability of collisions of the photons with particles of thephosphor material. In some embodiments, the inclusion of the lightreflective material can potentially, for a given emission product colorand intensity, reduce phosphor material usage by 33% or more. Someembodiments of the invention concern devices in which the wavelengthconversion component that includes the phosphor material is provided“remote” to the light emitter to reduce the transfer of heat from thelight emitter to the phosphor material. In the context of thisapplication, “remote” and “remotely” means physically separated from, byfor example an air gap or light transmissive medium. In remote phosphordevices the phosphor material is distributed over a much greater areathan the area of the light emitting surface of the light emitter. Inaccordance with some embodiments of the invention, the area over whichthe phosphor material and light reflective material is distributed is atleast fifty times the light emitting area of the light emitter.Moreover, in some embodiments, the wavelength conversion component islocated at a distance of at least 5 mm from the light emitter and ispreferably separated by a gap, e.g. an air gap. Separating the phosphormaterial from the solid-state emitter reduces the transfer of heat tothe phosphor material and reduces thermal degradation of the phosphormaterial.

Advantageously, the light reflective materials employed in someembodiments has as high a reflectivity as possible and preferably has areflectance of at least 0.9. The light reflective material can comprisemagnesium oxide (MgO), titanium dioxide (TiO₂), barium sulfate (BaSO₄)or a combination thereof. Preferably the light reflective material has aparticle size in a range 0.01 μm to 10 μm, 0.01 μm to 0.1 μm or 0.1 μmto 1 μm.

The phosphor material in some embodiments preferably has a particle sizein a range from 2 to 60 μm and typically in a range 10 to 20 μm. It isbelieved to be advantageous in some embodiments for the light reflectivematerial particle size to be smaller than the phosphor material particlesize preferably by a factor of at least ten. The weight percent loadingof light reflective material to phosphor material in some embodimentscan be in a range 0.01% to 10%, 0.1% to 1% or 0.5% to 1%.

In one arrangement the wavelength conversion component comprises a lighttransmissive substrate on which the mixture of phosphor and lightreflective materials is provided as at least one layer. The componentcan be configured as a light transmissive window such that a proportionof blue light passing through the component will be converted to lightof a different color by the phosphor material. Alternatively thecomponent can be configured as a light guide (waveguide) and the mixtureof phosphor and light reflective materials provided on at least a partof a face of the substrate. The mixture of phosphor and light reflectivematerials is advantageously applied to the surface of the substrate byscreen printing. Alternatively the mixture can be deposited on thesubstrate by inkjet printing, spin coating or doctor blading. Preferablythe light transmissive substrate comprises an acrylic, a polycarbonate,an epoxy, a silicone or a glass.

In another arrangement the wavelength conversion component comprises alight transmissive substrate having the mixture of phosphor and lightreflective materials homogeneously distributed throughout its volume.Preferably the light transmissive substrate comprises an acrylic, apolycarbonate, an epoxy, a silicone or a glass.

In further arrangements the wavelength conversion component is lightreflective and can comprise a light reflective surface on which themixture of phosphor and light reflective materials is provided as atleast one layer. The mixture of phosphor and light reflective materialscan be applied to the light reflective surface by screen printing, spincoating or doctor blading. The light reflective substrate can compriseany light reflective surface and preferably has a reflectance of atleast 0.9. The light reflective surface can comprise a polished metallicsurface such as silver aluminum, chromium; a light reflective polymer, alight reflective paper or a light reflective paint.

In some embodiments, the at least one solid-state light emittercomprises an LED that is operable to generate blue light having a peakwavelength in a wavelength range 440 nm to 480 nm. Alternatively thesolid-state light emitter(s) can comprise a laser or laser diode.

According to a further aspect of some embodiments of the invention aphotoluminescence wavelength conversion component for a solid-statelight emitting device comprises a mixture of particles of at least onephoto-luminescent material and particles of a light reflective material,wherein the mixture of at least one phosphor material and lightreflective material are distributed over an area of at least 0.8 cm².The light reflective material preferably has as high a reflectance aspossible, preferably at least 0.9, and can comprise particles of MgO,TiO₂, BaSO₄ or a combination thereof. Preferably the light reflectivematerial has a particle size in a range 0.01 μm to 10 μm; 0.01 μm to 1μm or 0.1 to 1 μm.

The phosphor material(s) preferably comprise an inorganic material suchas for example an orthosilicate, nitride, sulfate, oxy-nitride,oxy-sulfate or garnet (YAG) material has a particle size in a range 2 μmto 60 μm and more particularly 10 μm to 20 μm.

Advantageously the weight percent loading of light reflective materialto phosphor material is in a range 0.01% to 10%; 0.01% to 1%; 0.1% to 1%or 0.5% to 1%.

The wavelength conversion component can be light transmissive or lightreflective. Where the component is light transmissive the component canbe configured as a light transmissive window such that a proportion ofblue light passing through the component will be converted to light of adifferent color by the phosphor material(s). Alternatively the componentcan be configured as a light guide (waveguide) and the mixture ofphosphor and light reflective materials provided on at least a part of aface of the substrate or provided a layer in proximity the face. Themixture of phosphor material and light reflective material can be (a)provided as one or more layers on at least a part of the surface of thecomponent or (ii) homogeneously distributed throughout the volume of thelight transmissive substrate. The light transmissive substrate cancomprise a light transmissive polymer such as an acrylic, apolycarbonate, an epoxy or a silicone or a glass. Where the wavelengthconversion component is light reflective, the wavelength conversioncomponent can comprise a light reflective surface on which the mixtureof phosphor material and light reflective material is provided as one ormore layers. The light reflective surface can comprise any surface witha high reflectance, preferably at least 0.9, including a metal surfaceof silver, aluminum, chromium, a light reflective polymer, a lightreflective paper or card, a light reflective paint. For ease offabrication the mixture of phosphor material and light reflectivematerial can be provided on the component by printing, preferably screenprinting or inkjet printing; spin coating or doctor blading.

In some embodiments, the light reflective/scattering material that isutilized within the wavelength conversion component has a particle sizethat is selected such that the particles will scatter blue lightrelatively more than they will scatter light generated by the phosphormaterials. For example, the light reflective particle size may beselected such that the particles will scatter blue light relatively atleast twice as much as they will scatter light generated by the at leastone phosphor material. This ensures that a higher proportion of the bluelight emitted from the wavelength conversion layer will be scattered,thereby increasing the probability of the photon interacting with aphosphor material particle and resulting in the generation ofphoto-luminescent light. At the same time phosphor generated light canpass through with a lower probability of being scattered.

The light reflective/scattering material may be embodied in a separatelayer that is adjacent to, or near, a layer that includes the phosphormaterial. The separate light reflective layer may be used instead of,and/or in addition to, mixing light reflective/scattering material intothe same layer as the phosphor material. Either the same, or differentreflective materials, may be used in the separate light reflective layerfrom the light reflective material that is mixed with the phosphormaterial.

The embodiments of the invention may be applied to wavelength conversioncomponents having any suitable shape, whether planar orthree-dimensional and enveloping some additional volume.

According to a yet further aspect of some embodiments of the invention,a light emitting sign comprises at least one solid-state light emitteroperable to generate blue light and a photoluminescence signage surfacecomprising a light transmissive substrate having a mixture of particlesof at least one phosphor material and particles of a light reflectivematerial distributed over an area of at least 100 cm². The mixture ofphosphor material and light reflective material can be configured as apattern to define an image, picture, letter, numeral, device, pattern orother signage information. Alternatively, as for example is required forchannel lettering, the shape of the signage surface can be configured todefine signage information.

Where the sign is backlit, that is the light emitter is located behindthe signage surface which is configured as a light transmissive windowsuch that a proportion of blue light passing through the component willbe converted to light of a different color by the phosphor material, thesignage surface is preferably located at a distance of at least 5 mmfrom the light emitter. Alternatively the sign can be edge lit and thesubstrate configured as a light guide and the mixture of phosphormaterial and light reflective material are provided on at least a partof a light emitting face of the light guide. In some cases, thesubstrate will be planar and light can coupled into the light guide fromone or more edges of the substrate.

The light reflective material in some embodiments has a particle size0.01 μm to 10 μm; 0.01 μm to 1 μm or 0.1 μm to 1 μm whilst the phosphormaterial has a particle size of 2 μm to 60 μm and preferably 10 μm to 20μm. The weight percent loading of light reflective material to phosphormaterial can be in a range 0.01% to 10%; 0.01% to 1%; 0.1% to 1% or 0.5%to 1%. The light reflective material can comprise MgO, TiO₂, BaSO₄ or acombination thereof.

The mixture of phosphor material and light reflective material isadvantageously provided on the substrate by screen printing.Alternatively it can be deposited on the substrate by inkjet printing,spin coating or doctor blading.

The light transmissive substrate can comprise any light transmissivematerial including an acrylic, a polycarbonate, an epoxy, a silicone anda glass.

According to yet another aspect of the invention a photoluminescencesignage surface for a solid-state light emitting sign comprises a lighttransmissive substrate having a mixture of particles of at least onephosphor material and particles of a light reflective materialdistributed over an area of at least 100 cm².

The signage surface can be configured as a light transmissive windowsuch that a proportion of blue light passing through the component willbe converted to light of a different color. Alternatively the substratecan be configured as a light guide and the mixture of phosphor materialand light reflective material provided on, or in proximity to, at leasta part of a light emitting face of the light guide.

The light reflective material can have a particle size 0.01 μm to 10 μm;0.01 μm to 1 μm or 0.1 to 1 μm. The phosphor material may have aparticle size 2 μm to 60 μm and preferably in a range 10 μm to 20 μm. Insome embodiments, the weight percent loading of light reflectivematerial to phosphor material can be in a range 0.01% to 10%; 0.01% to1%; 0.1% to 1% or 0.5% to 1%. The light reflective material can compriseMgO, TiO₂, BaSO₄ or a combination thereof.

Preferably the mixture of phosphor material and light reflectivematerial is provided on the substrate by screen printing. Alternativelyit can be deposited by inkjet printing, spin coating or doctor blading.

The light transmissive substrate can comprise an acrylic, apolycarbonate, an epoxy, a silicone and a glass.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the present invention is better understood solid-statelight emitting devices and signs in accordance with embodiments of theinvention will now be described, by way of example only, with referenceto the accompanying drawings in which:

FIG. 1 is schematic representation of an LED-based light emitting devicein accordance with an embodiment of the invention;

FIG. 2 is a schematic illustrating the principle of operation of a knownlight emitting device;

FIG. 3 is a schematic illustrating the principle of operation of thelight emitting device of FIG. 1;

FIG. 4 is a plot of emission intensity versus chromaticity CIE x for anLED-based light emitting device in accordance with the invention fordifferent weight percent loadings of light reflective material;

FIG. 5 is a schematic representation of an LED-based light emittingdevice in accordance with an alternative embodiment of the invention;

FIG. 6 is a schematic representation of an LED-based light emittingdevice in accordance with a another embodiment of the invention;

FIG. 7 is a schematic representation of an LED-based light emittingdevice in accordance with a further embodiment of the invention;

FIG. 8 is a schematic illustrating the principle of operation of thelight emitting device of FIG. 7;

FIG. 9 is a schematic of a phosphor wavelength conversion component inaccordance with an embodiment of the invention;

FIG. 10 is a schematic of a phosphor wavelength conversion component inaccordance with another embodiment of the invention;

FIG. 11 shows plots of relative light scattering versus lightdiffractive particle size (nm) for red, green and blue light;

FIG. 12 illustrates an LED-based light emitting device in accordancewith a further embodiment of the invention; and

FIG. 13 illustrates a cross-sectional view of the LED-based lightemitting device of FIG. 12 in accordance with a further embodiment ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

Some embodiments of the invention are directed to light emitting devicescomprising one or more solid-state light emitters, typically LEDs, thatis/are operable to generate excitation light (typically blue) which isused to excite a wavelength conversion component containing particles ofa photoluminescence materials (e.g. phosphor materials), such as a bluelight excitable phosphor material. Additionally the wavelengthconversion component comprises particles of a light reflective material(also referred to herein as “light scattering material”) that isincorporated with the phosphor material to enhance photoluminescencelight generation by the phosphor material. The enhanced light generationresults from the light reflective material increasing the number ofcollisions of light generated by the light emitter(s) with particles ofthe phosphor material. The net result is a decrease in phosphor materialusage for the light emitting devices.

For the purposes of illustration only, the following description is madewith reference to photoluminescence material embodied specifically asphosphor materials. However, the invention is applicable to any type ofany type of photoluminescence material, such as either phosphormaterials or quantum dots. A quantum dot is a portion of matter (e.g.semiconductor) whose excitons are confined in all three spatialdimensions that may be excited by radiation energy to emit light of aparticular wavelength or range of wavelengths. As such, the invention isnot limited to phosphor based wavelength conversion components unlessclaimed as such. In addition, throughout this patent specification, likereference numerals are used to denote like parts.

FIG. 1 shows a schematic representation of an LED-based white lightemitting device 10 in accordance with an embodiment of the invention.The device 10 comprises a blue light emitting LED 12 and aphotoluminescence wavelength conversion component 14 located remote tothe LED. As shown the wavelength conversion component 14 can comprise alight transmissive window (substrate) 16 having, on at least one face, aphosphor conversion layer 18. The phosphor conversion layer 18 comprisesa mixture of particles of a blue light excitable phosphor material 20,particles of a light reflective material 22 and a light transmissivebinder material 24. The light transmissive window 16 can comprise anylight transmissive material such as a polymer material for example apolycarbonate, acrylic, silicone or epoxy or a glass such as a quartzglass. Typically, for ease of fabrication, the light transmissive window16 is planar often disc-shaped in form though it can be square,rectangular or other shapes depending on the intended application. Wherethe light transmissive window is disc-shaped the diameter can be betweenabout 1 cm and 10 cm that is an optical aperture of area between 0.8 cm²and 80 cm². In alternative embodiments it is envisioned that the lighttransmissive window 16 comprise an optical component that directs lightin a selected direction such as a convex or concave lens. To reduce thetransfer of heat from the LED 12 to the wavelength conversion component14, in particular heat transfer to the phosphor material, the wavelengthconversion component is located remote to the LED, physically separated,by a distance L of at least 5 mm. Embodiments of the present inventionconcern devices in which the wavelength conversion component and moreimportantly the phosphor material is provided remote to the LED toreduce the transfer of heat from the light emitter to the phosphormaterial. In the context of this application remote means physicallyseparated from by for example an air gap or light transmissive medium.It will be appreciated that in remote phosphor devices the phosphormaterial is distributed over a much greater area (e.g. 0.8 cm² to 80cm²) than the area of the light emitting surface of the LED (e.g. 0.03cm²). Typically the phosphor material is distributed over an area thatis at least fifty times, typically at least 100 times, the lightemitting area of the LED.

The blue LED 12 can comprise a GaN-based (gallium nitride-based) LEDthat is operable to generate blue light 26 having a peak wavelength λ₁in a wavelength range 440 nm to 480 nm (typically 465 nm). The blue LED12 is configured to irradiate the wavelength conversion component 14with blue excitation light 26 whereat a proportion is absorbed by thephosphor material 20 which in response emits light 28 of a differentwavelength λ₂, typically yellow-green in color for a cold white lightemitting device. The emission product 30 of the device 10 which isconfigured to appear white in color comprises the combined light 26emitted by the LED and the light 28 generated by the phosphor material20.

The phosphor material 20 and light reflective material 22, which are inpowder form, are thoroughly mixed in known proportions with the lighttransmissive binder material 24 such as a polymer material (for examplea thermally or UV curable silicone or an epoxy material) or a clear inksuch as for example Nazdar's® UV curable litho clear overprint PSLC-294.The mixture is applied to the face of the window 16 as one or morelayers of uniform thickness. In a preferred embodiment the mixture isapplied to the light transmissive window by screen printing and thethickness t of the layer controlled by the number of printing passes. Aswill be apparent to those skilled in the art the phosphor/reflectivematerial mixture can be applied using other methods including inkjetprinting, spin coating or sweeping the mixture over the surface using ablade such as a squeegee (e.g. doctor blading).

It is envisioned in further embodiments to incorporate the mixture ofphosphor and light reflective material mixture within the lighttransmissive window. For example the phosphor and light reflectivematerial mixture can be mixed with a light transmissive polymer and thepolymer/phosphor mixture extruded or injection molded to form thewavelength conversion component 14 with the phosphor and lightreflective material homogeneously distributed throughout the volume ofthe component.

Locating the phosphor material remote to the LED provides a number ofbenefits namely reduced thermal degradation of the phosphor material.Additionally compared with devices in which the phosphor material isprovided in direct contact with the light emitting surface of the LEDdie, providing the phosphor material remotely reduces absorption ofbackscattered light by the LED die. Furthermore locating the phosphorremotely enables generation of light of a more consistent color and/orCCT since the phosphor material is provided over a much greater area ascompared to providing the phosphor directly to the light emittingsurface of the LED die.

The phosphor material can comprise an inorganic or organic phosphor suchas for example silicate-based phosphor of a general compositionA₃Si(O,D)₅ or A₂Si(O,D)₄ in which Si is silicon, O is oxygen, Acomprises strontium (Sr), barium (Ba), magnesium (Mg) or calcium (Ca)and D comprises chlorine (Cl), fluorine (F), nitrogen (N) or sulfur (S).Examples of silicate-based phosphors are disclosed in U.S. Pat. No.7,575,697 “Europium activated silicate-based green phosphor” (assignedto Intematix Corp.), U.S. Pat. No. 7,601,276 “Two phase silicate-basedyellow phosphor” (assigned to Intematix Corp.), U.S. Pat. No. 7,601,276“Silicate-based orange phosphor” (assigned to Intematix Corp.) and U.S.Pat. No. 7,311,858 “Silicate-based yellow-green phosphor” (assigned toIntematix Corp.). The phosphor can also comprise an aluminate-basedmaterial such as is taught in our co-pending patent applicationUS2006/0158090 “Aluminate-based green phosphor” and patent U.S. Pat. No.7,390,437 “Aluminate-based blue phosphor” (assigned to Intematix Corp.),an aluminum-silicate phosphor as taught in co-pending applicationUS2008/0111472 “Aluminum-silicate orange-red phosphor” or anitride-based red phosphor material such as is taught in co-pending U.S.patent application Ser. No. 12/632,550 filed Dec. 7, 2009. It will beappreciated that the phosphor material is not limited to the examplesdescribed herein and can comprise any phosphor material includingnitride and/or sulfate phosphor materials, oxy-nitrides and oxy-sulfatephosphors or garnet materials (YAG).

The phosphor material comprises particles that are generally sphericalin form with a diameter of 10 μm to 20 μm and typically of order 15 μm.The phosphor material can comprise particles of a size 2 μm to 60 μm.

The light reflective material 22 comprises a powdered material with ahigh reflectivity typically a reflectance of 0.9 or higher. The particlesize of the light reflective material is typically in a range 0.1 μm to10 μm and in a preferred embodiment is within a range 0.1 μm to 10 μm.The weight percent loading of light reflective material to phosphormaterial is in a range 0.1% to 10% and in a preferred embodiment in arange 1% to 2%. Examples of light reflective materials include magnesiumoxide (MgO), titanium dioxide (TiO₂), barium sulfate (BaSO₄) andcombinations thereof. The light reflective material can also comprise awhite ink such as for example Norcote International Inc's super whiteink GN-027SA which already includes particles of a highly lightreflective material, typically TiO₂.

Before describing operation of the device of the invention, operation ofa known light emitting device will be described with reference to FIG. 2which shows a schematic of a cool white LED-based light emitting devicethat utilizes phosphor wavelength conversion. In common with the deviceof the invention the known device includes a wavelength conversioncomponent 18 that includes phosphor material particles 20 homogeneouslydistributed throughout the volume of a light transmissive binder 24.Unlike the device of the invention the known devices do not includeparticles of a light reflective material. In operation blue light 26from the LED is transmitted by the light transmissive binder 24 until itstrikes a particle of phosphor material. It is believed that on averageas little as 1 in a 10,000 interactions of a photon with a phosphormaterial particle results in absorption and generation ofphotoluminescence light. The majority, about 99.99%, of interactions ofphotons with a phosphor particle result in scattering of the photon. Dueto the isotropic nature of the scattering process on average half thescattered photons will be in a direction back towards the LED. Testsindicate that typically about 10% of the total incident blue light isscattered and emitted from the wavelength conversion component in adirection back towards the LED. For a cool white light emitting devicethe amount of phosphor material is selected to allow approximately 10%of the total incident blue light to be emitted through the window andcontribute to the emission product. The majority, approximately 80%, ofthe incident light is absorbed by the phosphor material and re-emittedas photoluminescence light 28. Due to the isotropic nature ofphotoluminescence light generation, approximately half of the light 28generated by the phosphor material will be emitted in a directiontowards the LED. As a result up to (↑) 40% of the total incident lightwill be emitted as light 28 of wavelength λ₂ and contributes to theemission product 30 whilst up to (↑) 40% of the total incident lightwill be emitted as light 28 of wavelength λ₂ in a direction back towardsthe LED. Typically light that is emitted towards the LED is re-directedby a reflector (not shown) to increase the overall efficacy of thedevice.

Operation of a cool white light emitting device 10 in accordance withsome embodiments of the invention is now described with reference toFIG. 3 which shows a schematic of operation of the device of FIG. 1. Theoperation of the device of the invention is similar to that of FIG. 2but additionally includes reflection or scattering of light (ofwavelengths λ₁ and λ₂) by the particles of the lightreflective/scattering material 22. By including particles of a lightreflective material with the phosphor material this can reduce theamount of phosphor material required to generate a given color emissionproduct, e.g. by up to 33% in some embodiments. It is believed that theparticles of light reflective material increase the probability ofphotons striking a particle of phosphor material and thus for anemission product of a given color less phosphor material is required.

FIG. 4 is a plot of emission intensity versus chromaticity CIE x for alight emitting device in accordance with some embodiments of theinvention for weight percent loadings of light reflective material of♦−0%, ▪−0.4%, ▴−1.1% and −2%. The data are for screen printed phosphorconversion layers in which the binder material comprises Nazdar's® UVcurable litho clear overprint PSLC-294 and the phosphor materialcomprises Intematix Corporation's phosphor EY4453 with an averageparticle size of 15 μm. The ratio of phosphor material to clear ink isin a proportion of 2:1 by weight. The light reflective materialcomprises Norcote International Inc's super white ink GN-027SA. Thefigures for loading of light reflective material refer to weight percentof super white ink to clear ink. The smaller reference numeralsassociated with each data point indicate the number ‘n’ of print passesused to form the phosphor layer. It will be appreciated that the numberof print passes is directly proportional to the thickness of thephosphor layer 18 and quantity of phosphor. The ovals 32, 34, 36, 38 areused to group data points for emission products that have substantiallythe same intensity and CIE x values. For example oval 32 indicates thatan emission product of similar intensity and color can be produced for aphosphor conversion layers 18 comprising i) 3 print passes without lightreflective material and ii) 2 print passes with a 2% loading of lightreflective material. These data indicate that by the inclusion of a 2%weight loading of light reflective material it is possible to generatethe same color and intensity of light using a phosphor conversion layer18 that comprises about 33% less phosphor material. Oval 34 indicatesthat the same intensity and color of emission product is produced for aphosphor conversion layer comprising i) 4 print passes without lightreflective material and ii) 3 print passes with a 0.4% loading of lightreflective material. These data indicate that for this embodiment, bythe inclusion of a 0.4% weight loading of light reflective material, thesame color and intensity of light can be produced using a phosphorconversion layer comprising about 25% less phosphor. Oval 36 indicatesthat the same intensity and color of emission product is produced for aphosphor conversion layer comprising i) 4 print passes without lightreflective material and ii) 3 print passes with a 1.1% loading of lightreflective material. These data indicate that by the inclusion of a 1.1%weight loading of light reflective material the same color and intensityof light can be produced using a phosphor conversion layer comprisingabout 25% less phosphor. Oval 38 indicates that the same intensity andcolor of emission product is produced for a phosphor conversion layercomprising i) 4 print passes with a 0.4% weight loading of lightreflective material and ii) 3 print passes with a 2% weight loading oflight reflective material. These data indicate by the inclusion of a0.4% weight loading of light reflective material that the same color andintensity of light can be produced using a phosphor conversion layercomprising about 25% less phosphor. Points 40 (n=4, 1.1% loading) and 42(n=4, 2% loading) suggest that a saturation point exists above which anincrease in light reflective material loading results in a decrease inemission intensity with little effect on the color.

FIG. 5 is a schematic representation of an LED-based white lightemitting device 10 in accordance with another embodiment of theinvention. In this embodiment the light transmissive substrate 16 isconfigured as a light guide (waveguide) and the phosphor conversionlayer 18 is provided over one face of the substrate, the light emittingface. Typically the substrate 16 is substantially planar and can bedisc-shaped, square, rectangular or other shapes depending on theapplication. Where the substrate is disc-shaped the diameter cantypically be between about 5 cm and 30 cm corresponding to a lightemitting face of area of between about 20 cm² and about 700 cm². Wherethe substrate is square or rectangular in form the sides can typicallybe between about 5 cm and 40 cm corresponding to a light emitting faceof between about 80 cm² and about 5000 cm². On the non-light emittingface (the lower face as illustrated) of the substrate 16 a layer oflight reflective material 44 can be provided to prevent the emission oflight from the rear of the device. The reflective material 44 cancomprise a metallic coating such as chromium or a glossy white materialsuch as a plastics material or paper. To minimize light being emittedfrom the edges of the substrate, the edges of the substrate preferablyinclude a light reflective surface (not shown). One or more blue LEDs 12are configured to couple blue light 26 into one or more edges of thesubstrate 16. In operation light 26 coupled into the substrate 16 isguided throughout the entire volume of the substrate 16 by totalinternal reflection. Light 26 striking the light emitting face of thesubstrate at angles above a critical angle will be emitted through theface and into the phosphor wavelength conversion layer 18. Operation ofthe device is the same as that described with reference to FIG. 3. Asindicated in FIG. 5 phosphor generated light 46 emitted in directionsaway from the light emitting face can re-enter the substrate 16 and willeventually be emitted through the light emitting face by being reflectedby the light reflective layer 44. The final illumination product 30emitted from the device is the combination of the blue light 26generated by the LED and wavelength converted light 28 generated by thephosphor wavelength conversion layer 18.

FIG. 6 is a schematic representation of an alternative LED-based whitelight emitting device 10 in which the light transmissive substrate 16 isconfigured as a light guide (waveguide). In this embodiment the phosphorconversion layer 18 is provided on the face of the substrate that isopposite to the light emitting face and the light reflective layer 44 isprovided over the phosphor conversion layer 18.

FIG. 7 shows a schematic representation of an LED-based white lightemitting device 10 in accordance with a further embodiment of theinvention. In this embodiment the wavelength conversion component 14 islight reflective and comprises a light reflective surface 48 on whichthe phosphor conversion layer 18 is applied. As shown the lightreflective surface 48 can comprise a parabloidal surface though it cancomprise any surface including planar, convex and concave surfaces. Tomaximize light emission from the device, the light reflective surface isas reflective as possible and preferably has a reflectance of at least0.9. The light reflective surface can comprise a polished metallicsurface such as silver, aluminum, chromium; a light reflective polymer,a light reflective paper or a light reflective paint. To assist in thedissipation of heat the light reflective surface is preferably thermallyconductive.

Operation of the light emitting device of FIG. 7 is illustrated in FIG.8 and is not described in detail as it is similar to that of FIG. 3.However it is to be appreciated that since on average up to half of theLED light 26 will travel through the phosphor conversion layer twice,the thickness of the phosphor conversion layer 18 can be of up to half,i.e. t/2, compared to arrangements with a light transmissive wavelengthconversion component (FIGS. 1 and 5). As a result of providing thephosphor material on a light reflective surface the same color ofemission product can be achieved with a further potential reduction ofup to about 50% in phosphor material usage. It will be appreciated thatthe embodiment of FIG. 6 is similar in terms of operation to that ofFIG. 7 with the light transmissive substrate 16 being used to guide LEDlight 26 to the phosphor conversion layer 18.

Whilst the invention has been described in relation to light emittingdevices the principles of the invention also apply to solid-state lightemitting signage that utilize photoluminescence wavelength conversion togenerate a desired color of emitted light such as those disclosed inco-pending United States patent application US 2007/0240346 A1, to Li etal., the specification of which is incorporated herein by way ofreference thereto. It will be appreciated that in such light emittingsigns the wavelength conversion component 14 can be used as thephotoluminescence signage surface to generate signage information of adesired color. The mixture of phosphor material and light reflectivematerial can be configured as a pattern to define an image, picture,letter, numeral, device, pattern or other signage information on thelight transmissive substrate. Alternatively, as for example is requiredfor channel lettering, the shape of the signage surface, that is thelight transmissive substrate, can be configured to define signageinformation. The invention is particularly advantageous in signageapplications where the area of the signage surface is many hundreds ofsquare centimeters requiring the phosphor material to be distributedover a minimum area of 100 cm² (10 cm by 10 cm) and more typically overmany hundreds or even thousands of square centimeters.

The signs can be backlit, that is the LEDs are located behind thesignage surface within for example a light box, and the signage surfaceprovided overlaying the light box opening. Typically the signage surfaceis located at a distance of at least about 5 mm from the LEDs.Alternatively the sign can be edge lit and the light transmissivesubstrate configured as a light guide and the mixture of phosphormaterial and light reflective material provided on at least a part of alight emitting face of the light guide.

In some embodiments, the light reflective material comprises titaniumdioxide (TiO₂) though it can comprise other materials such as bariumsulfate (BaSO₄), magnesium oxide (MgO), silicon dioxide (SiO₂) oraluminum oxide (Al₂O₃). In some embodiments, the light reflectivematerial has an average particle size in a range 1 μm to 50 μm and morepreferably in a range 10 μm to 20 μm.

In some embodiments, the light reflective/scattering material that isutilized within the wavelength conversion component has a particle sizethat is selected such that the particles will scatter excitation(typically blue) light relatively more than they will scatter lightgenerated by the photoluminescence (phosphor) material(s). For example,the light reflective particle size may be selected such that theparticles will scatter excitation light relatively at least twice asmuch as they will scatter light generated by the at least one phosphormaterial. This ensures that a higher proportion of the blue excitationlight will be scattered, increasing the probability of the photoninteracting with a phosphor material particle and resulting in thegeneration of photoluminescence light. At the same time phosphorgenerated light can pass through with a lower probability of beingscattered.

Since this approach can further increase the probability of blue photonsinteracting with a phosphor material particle, less phosphor material isrequired to generate a selected emission color. This arrangement canalso increase luminous efficacy of the wavelength conversioncomponent/device. In some embodiments employing blue (400 nm to 480 nm)excitation light, the light reflective material has an average particlesize of less than about 150 nm and typically has an average particlesize in a range 100 nm to 150 nm.

The light reflective/scattering material (i.e. for preferentiallyscattering blue light) may be embedded within the same layer of materialas the phosphor material.

Alternatively, light reflective/scattering material may be placed onto aseparate layer that is adjacent to or nearby the layer having thephosphor material. For example, In accordance with some embodiments ofthe invention and as shown in FIG. 9 the wavelength conversion component136 comprises, in order, a light transmissive substrate 142, a lightreflective layer 144 containing light reflective particles and awavelength conversion layer 146 containing a mixture of one or morephosphor (photoluminescence) and light reflective materials. As can beseen in FIG. 9 the wavelength conversion component 136 is configuredsuch that in operation the wavelength conversion layer 146 faces theLEDs. In accordance with some embodiments of the invention thewavelength conversion component 136 can comprises, in order, a lighttransmissive substrate 142, a light reflective layer 144 containinglight reflective particles and a wavelength conversion layer 146containing a one or more phosphor (photoluminescence) materials.

The light transmissive substrate 142 can be any material that issubstantially transmissive to light in a wavelength range 380 nm to 740nm and can comprise a light transmissive polymer such as a polycarbonateor acrylic or a glass such as a borosilicate glass. The substrate 142 insome embodiments comprise a planar circular disc of diameter φ=62 mm andthickness t₁ which is typically 0.5 mm to 3 mm. In other embodiments thesubstrate can comprise other geometries such as being convex or concavein form such as for example being dome shaped or cylindrical.

The light diffusing layer 144 comprises a uniform thickness layer ofparticles of a light reflective material, preferably titanium dioxide(TiO₂). In alternative arrangements the light reflective material cancomprise barium sulfate (BaSO₄), magnesium oxide (MgO), silicon dioxide(SiO₂), aluminum oxide (Al₂O₃) or a powdered material with as high areflectivity as possible, typically a reflectance of 0.9 or higher. Thelight reflective material powder is thoroughly mixed in knownproportions with a light transmissive liquid binder material to form asuspension and the resulting mixture deposited onto the face of thesubstrate 142 preferably by screen printing to form a uniform layer ofthickness t₂ (typically in a range 10 μm to 75 μm) that covers theentire face of the substrate. The quantity of light diffracting materialper unit area in the light diffusing layer 144 will typically in a range10 μg·cm⁻² to 5 mg·cm⁻².

Whilst screen printing is a preferred method for depositing the lightdiffusing layer 144, it can be deposited using other techniques such asfor example slot die coating, spin coating, roller coating, drawdowncoating or doctor blading. The binder material can comprise a curableliquid polymer such as a polymer resin, a monomer resin, an acrylic, anepoxy (polyepoxide), a silicone or a fluorinated polymer. It isimportant that the binder material is, in its cured state, substantiallytransmissive to all wavelengths of light generated by the phosphormaterial(s) and the LEDs and preferably has a transmittance of at least0.9 over the visible spectrum (380 nm to 800 nm). The binder material ispreferably U.V. curable though it can be thermally curable, solventbased or a combination thereof. U.V. or thermally curable binders can bepreferable because, unlike solvent-based materials, they do not “outgas”during polymerization. In one arrangement the average particle size ofthe light diffractive material is in a range 5 μm to 15 μm though asdescribed above it can be in a nanometer range (nm) and isadvantageously in a range 100 nm to 150 nm. The weight percent loadingof light reflective material to liquid binder is typically in a range 7%to 35%.

The wavelength conversion layer 146 is deposited in direct contact withthe light diffusing layer 144 that is without any intervening layers orair gaps. The phosphor material, which is in powder form, is thoroughlymixed in known proportions with a liquid light transmissive bindermaterial to form a suspension and the resulting phosphor composition,“phosphor ink”, deposited directly onto the reflective layer 144. Thewavelength conversion layer is preferably deposited by screen printingthough other deposition techniques such as slot die coating, spincoating or doctor blading can be used. To eliminate an optical interfacebetween the wavelength conversion and reflective layers 146, 144 and tomaximize the transmission of light between layers, the same liquidbinder material is preferably used to fabricate both layers; that is, apolymer resin, a monomer resin, an acrylic, an epoxy, a silicone or afluorinated polymer.

A further example of a phosphor wavelength conversion component 136 inaccordance with the invention is illustrated in FIG. 10. In common withthe wavelength conversion component of FIG. 9 the component comprises alight transmissive substrate 142, a light diffusing layer 144 and awavelength conversion layer 146. In accordance with the invention thelight diffusing and wavelength conversion layers 144, 146 are depositedin direct contact with one another. Again in operation the component isconfigured such that the wavelength conversion component is configuredsuch that the wavelength conversion layer 146 faces the LEDs.

In operation blue excitation light 128 generated by the LEDs travelsthrough the wavelength conversion layer 146 until it strikes a particleof phosphor material. It is believed that on average as little as 1 in10,000 interactions of a photon with a phosphor material particleresults in absorption and generation of photo luminescence light 138.The majority, about 99.99%, of interactions of photons with a phosphorparticle result in scattering of the photon. Due to the isotropic natureof the scattering process on average half of the photons will scatteredin a direction back towards the LEDs. Tests indicate that typicallyabout 10% of the total incident blue light 128 is scattered and emittedfrom the wavelength conversion component 136 in a direction back towardsthe LEDs. For a cool white light emitting device the amount of phosphormaterial is selected to allow approximately 10% of the total incidentblue light to be emitted from the wavelength conversion component andcontribute to the emission product 140. The majority, approximately 80%,of the incident light is absorbed by the phosphor material andre-emitted as photo luminescence light 138. Due to the isotropic natureof photo luminescence light generation, approximately half of the light138 generated by the phosphor material will be emitted in a directiontowards the LED. As a result only up to about 40% of the total incidentlight will be emitted as light 138 of wavelength λ₂ and contributes tothe emission product 138 with the remaining (up to about 40%) of thetotal incident light being emitted as light 138 of wavelength λ₂ in adirection back towards the LED. Light emitted towards the LEDs from thewavelength conversion component 136 is re-directed by the lightreflective surfaces of a reflection chamber to contribute to theemission product and to increase the overall efficiency of the device.

The addition of a light diffusing layer 144 composed of particles of alight reflective material can substantially reduce the quantity ofphosphor material required to generate a selected color of emittedlight. The diffusing layer 144 increases the probability that a photonwill result in the generation of photoluminescence light by reflectinglight back into the wavelength conversion layer 146. Inclusion of areflective layer in direct contact with the wavelength conversion layercan reduce, the quantity of phosphor material required to generate agiven color emission product, e.g. by up to 40% in some embodiments.

Therefore, it is envisioned to configure the light diffusing layer suchthat it selectively scatters blue excitation light generated by the LEDsmore than it scatters light generated by the phosphor material. Such alight diffusing layer ensures that a higher proportion of the blue lightemitted from the wavelength conversion layer will be scattered anddirected by the light reflective material back into the wavelengthconversion layer increasing the probability of the photon interactingwith a phosphor material particle and resulting in the generation ofphotoluminescence light. At the same time phosphor generated light canpass through the diffusing layer with a lower probability of beingscattered. Since the diffusing layer increases the probability of bluephotons interacting with a phosphor material particle less phosphormaterial is needed to generate a selected emission color.

In addition, such an arrangement can also increase luminous efficacy ofthe wavelength conversion component/device. By appropriate selection ofthe average particle size of the light scattering material it ispossible to configure the light diffusing layer such that it scattersblue light more readily than other colors, namely green and red. FIG. 11shows plots of relative light scattering versus TiO₂ average particlesize (nm) for red, green and blue light. As can be seen from FIG. 11TiO₂ particles with an average particle size of 100 nm to 150 nm aremore than twice as likely to scatter blue light (450 nm to 480 nm) thanthey will scatter green light (510 nm to 550 nm) or red light (630 nm to740 nm). For example TiO₂ particles with an average particle size of 100nm will scatter blue light nearly three times (2.9=0.97/0.33) more thanit will scatter green or red light. For TiO₂ particles with an averageparticle size of 200 nm these will scatter blue light over twice(2.3=1.6/0.7) as much as they will scatter green or red light. Inaccordance with some embodiments of the invention the light diffractiveparticle size is preferably selected such that the particles willscatter blue light relatively at least twice as much as light generatedby the phosphor material(s). The concept of a wavelength conversioncomponent including a light reflective layer comprised of lightreflective particles that preferentially scatter light corresponding towavelengths generated by the LEDs compared with light of wavelengthsgenerated by the phosphor material is considered inventive in its ownright.

Therefore, the light reflective/scattering material may be embodied in aseparate layer that is adjacent to, or near, a layer that includes thephosphor material. The separate light reflective layer may be usedinstead of, and/or in addition to, mixing light reflective/scatteringmaterial into the same layer as the phosphor material. Either the same,or different reflective materials, may be used in the separate lightreflective layer from the light reflective material that is mixed withthe phosphor material.

The inventive concepts disclosed herein may be applied to wavelengthconversion components that encompass any suitable shape. For example,consider the LED lighting device 200 illustrated in FIGS. 12 and 13which shows a solid-state light bulb for replacing an incandescent lightbulb.

The LED lighting device 200 comprises a lighting base 204 that includesa screw base 206. Screw base 206 is configured to fit within standardlight bulb sockets, e.g. implemented as a standard Edison screw base. Anenvelope 208 may extend around the upper portion of the LED lightingdevice 200. The envelope 208 is a light-transmissive material (e.g.glass or plastic) that provides protective and/or diffusive propertiesfor the LED lighting device 200.

LED lighting device 200 comprises a wavelength conversion component 202having an elongated dome shape that extends from a lighting base 204.The blue LED device 12 resides on the top surface of the lighting base204, beneath the wavelength conversion component 202. Thethree-dimensional nature of the wavelength conversion component 202creates a relatively large shape that surrounds the volume around andabove the LEDs 12. Using three-dimensional shapes for the wavelengthconversion component 202 in a lighting device 200 allows for certainfunctional advantages, such as the ability to perform light shaping forthe light emitted by the lighting device 200.

However, these types of three-dimensional shapes for the wavelengthconversion component 202 also correspond to a relatively large volumefor the wavelength conversion component which needs to be populated withadequate amounts of the phosphor materials. With prior art approaches, asignificantly large amount of phosphor material would therefore berequired to manufacture such wavelength conversion components 202.Embodiments of the invention may be employed to reduce the amount ofphosphor needed to manufacture such wavelength conversion components202. In particular, the wavelength conversion component 202 comprises amixture of phosphors and a reflective material. Since the reflectivematerial within the wavelength conversion component 202 has the propertyof scattering light, this reduces the amount of phosphor material thatis needed for the wavelength conversion component 202.

In some embodiments, a light diffusing layer (not shown) may be added tothe wavelength conversion component 202 (in addition to and/or insteadof the reflective material mixed with the phosphors) to reduce theamount of phosphor material required to manufacture the wavelengthconversion component 202. Any suitable material may be employed for thelight reflective material, such as light scattering particles that areselected to be small enough to more likely scatter blue light.

Therefore, what has been described is an improved approach forimplementing LED-based lighting devices and/or wavelength conversioncomponents that reduce the amount of photo-luminescent materials neededto manufacture such devices and components.

It will be appreciated that light emitting devices in accordance withthe invention are not limited to the exemplary embodiments described andthat variations can be made within the scope of the invention. Forexample whilst the invention has been described in relation to LED-basedlight emitting devices the invention also applies to devices based onother solid-state light emitters including solid-state lasers and laserdiodes.

What is claimed is:
 1. A light emitting device comprising: at least onesolid-state light emitter operable to generate excitation light; and aphotoluminescence wavelength conversion component comprising a mixtureof particles of at least one photoluminescence material and particles ofa light reflective material.